A mobile radio communication system, such as a UMTS (Universal Mobile Telecommunication System) type system, includes a mobile radio communication network communicating with mobile terminals or UEs (User Equipments) and with external networks. Traditionally, communications are facilitated using one or more radio base stations that provide radio coverage for one or more cell areas. Various standards for cellular mobile communications include procedures to handover different UEs from one cell to another depending on the experienced radio conditions (e.g. moving UEs). This is the case of 3GPP standards such as GSM, WCDMA, or LTE. The technology in this application may be applied to handover procedures within any cellular mobile communication technology that supports handover including handover from one radio access technology to a different radio access technology (e.g., IRAT HO).
FIG. 1 shows a simplified cellular radio communications system with three cells 1-3 each having an associated base station (BS). As a UE actively involved in a radio connection currently being serviced by cell 1 moves farther away from base station BS1 to cell 3 and base station BS3, the radio connection is handed over to cell 3, which then becomes the serving cell for that connection.
In a typical handover procedure, the UE measures the received radio signal power or quality from downlink transmissions from neighboring cells. When a candidate cell downlink transmission is received with a higher radio signal power or better radio signal quality than the current cell that is serving the UE, a handover procedure is initiated towards the better or best cell.
Due to the varying nature of the radio signals, it is possible that what appears to be an increase of the received radio signal power or quality of a target neighbor cell due to movement is actually a fast signal fluctuation that lasts for only a short period of time. Such fast signal changes typically do not follow a long term average trend of the path loss for a given UE movement pattern, and as a result, may create a series of handovers in a relatively short period of time which are often not beneficial or needed. Each handover has an associated “cost” due to the associated handover signaling, increased probability of the connection being dropped, possible adverse effects on higher layer protocols, e.g., that forward data packets from source cell to target cell, etc. Such a series of handovers occurring in a relatively short period of time is referred to as “handover oscillation.”
Because of the cost associated with each handover, handover oscillation control is desirable. Handover parameters may be used to reduce handover oscillations between different cells. Handover parameters are usually set manually by network operators. Optimal values for handover parameters differ from cell to cell, which presents a problem if such optimal values change with time due to variations in the UE movement pattern, changes in network deployment (e.g., antenna tilt, transmission power, etc.), addition of new cell sites, etc. As a result, operators set default values for the handover parameters and only adjust those default values if a problem is detected. Unfortunately, default handover parameter settings are often not optimal. With default handover parameter values, cells that experience a high number of handover oscillations may only be able to reduce those oscillations to a certain extent, and sometimes, not sufficiently. Cells that do not experience handover oscillations would benefit from having handover parameter settings with a faster reaction time because that would mean that the UEs would spend more time in the best serving cell, thereby reducing interference to neighboring cells. (Typically, the higher the value of the handover parameter, the slower the handover decision is taken, which means a longer time period when the UE is in a non-optimal cell).
Another issue relates to modifying handover parameters. Modifying handover parameters may lead to an increase of handover failures. In that case, it would be desirable for handover performance to be monitored if there is handover oscillation control.